feat(images): convert all images to WebP with 87% size reduction

- Convert 34 images (PNG/JPG) to WebP at quality 80
- Total savings: 21.6 MB → 2.8 MB (87% reduction)
- Add <picture> elements with WebP source + original fallback
- Add loading=lazy to all below-the-fold images
- Update lightbox to serve WebP images with error fallback

js/haus-schleusingen.js

@@ -46,6 +46,11 @@ $(function () {
   // Lightbox – gallery grid items
   $(document).on("click", ".grid-item", function () {
     var src = $(this).data("img") || $(this).find("img").attr("src");
+    $("#lightboxImg").on("error", function () {
+      // WebP fallback: try original format
+      var fallback = src.replace(/\.webp$/, src.endsWith('.webp') ? '.png' : '.jpg');
+      if ($(this).attr('src') !== fallback) $(this).attr('src', fallback);
+    });
     $("#lightboxImg").attr("src", src);
     $("#lightbox").addClass("open");
     $("body").css("overflow", "hidden");
@@ -54,6 +59,10 @@ $(function () {
   // Lightbox – floor plan images in Raumaufteilung
   $(document).on("click", ".floor-plan img[data-img]", function () {
     var src = $(this).data("img");
+    $("#lightboxImg").on("error", function () {
+      var fallback = src.replace(/\.webp$/, '.png');
+      if ($(this).attr('src') !== fallback) $(this).attr('src', fallback);
+    });
     $("#lightboxImg").attr("src", src);
     $("#lightbox").addClass("open");
     $("body").css("overflow", "hidden");